1. Field of the Invention
This invention relates to a semiconductor device having a MIS (Metal Insulator Semiconductor) structure using a polysilicon thin film as a channel layer.
2. Description of the Related Art
A TFT (Thin Film Transistor) serving as a driving element in an active matrix-type liquid-crystal display device, and a MOSFET, etc., are used as conventional polysilicon thin film semiconductor devices of a MIS structure. Here, a top-gate type polysilicon thin film semiconductor device (MOSFET) will be explained with reference to FIG. 1.
In the polysilicon thin film MOSFET of FIG. 1, a gate electrode 15G of a polysilicon film, etc. is provided on a predetermined region of a polysilicon film pattern 12 of a first conductivity type which is formed on a synthetic quartz substrate 11, with a gate insulating film (thermal oxide film) 14 interposed therebetween. A source region 13S of a second conductivity type and a drain region 13D of a second conductivity type are formed in predetermined upper portions of the pattern 12, and a channel-forming region 13C is interposed between the regions 13S and 13D, and formed under the gate electrode 15G. The source and drain regions 13S and 13D are respectively connected to a source electrode 15S of aluminum and a drain electrode 15D of the same. The electrodes 15S, 15D, and 15G are insulated from each other by means of an interlayer insulating film 16 of oxidized silicon. When a voltage is impressed on the gate electrode 15G, a channel layer of a second conductivity type will be formed in the channel-forming region 13C, thereby causing current between the source region 13S and drain region 13D.
In the above-described MOSFET or other semiconductor devices of a MIS structure having a polysilicon thin film serving as a channel layer, the mobility of carriers in the channel layer is larger ten - hundred times than in a semiconductor device having an amorphous silicon thin film serving as a channel layer, in which, for example, an island 12 in the MIS structure is an amorphous silicon thin film pattern.
However, the polysilicon thin film contains a great amount of silicon crystal grains and grain boundaries, and hence a great number of dangling bonds of silicon atoms inevitably exist in the grain boundaries. These dangling bonds trap carriers in the channel layer, so that the mobility of the carriers there is reduced. Further, the threshold voltage of the above semiconductor device is high. Therefore, the conventional polysilicon thin film semiconductor device is impractical.